Brucellosis is a zoonotic disease caused by members of the genus Brucella, which are Gram-negative, facultatively intracellular bacteria. Of the six well-recognized species of Brucella, B. melitensis, B. suis and B. abortus are highly virulent to humans. These 3 Brucella species are considered potential bioterror agents and they belong to NIAID Category B priority pathogens list. At present there is no vaccine available for human brucellosis. Cell-mediated immunity (CMI) and antibodies to the O polysaccharide (O antigen) of the lipopolysaccharide play important roles in acquired resistance against brucellosis. Attenuated, live Brucella strains such as B. abortus RB51 and 19, and B. melitensis Rev1 are being used as vaccines to control brucellosis in domestic animals. However, these live vaccines are virulent in humans. In general, all live vaccines possess an inherent safety risk. The overall hypothesis of this research project is that a gamma- irradiated, recombinant RB51 strain expressing low amounts of O antigen in its cytoplasm and simultaneously overexpressing the protective protein(s) of B. melitensis, B. suis and B. abortus would be a highly effective and safe vaccine for human brucellosis. We have previously demonstrated that the vaccine efficacy of strain RB51 can be enhanced significantly by overexpressing a Brucella protection protein or by expressing O antigen in its cytoplasm. Our recent research indicates that strain RB51 and its recombinants subjected to an appropriate dose of gamma radiation are unable to replicate but are still as efficient as live strains in inducing protective immune responses. In the current proposal, under Specific Aim 1, we will carryout experiments to identify common Brucella protective proteins by overexpressing selected proteins in strain RB51 and then checking for the induction of enhanced protection in mice against all 3 virulent Brucella species infections. Under Specific Aim 2, we will construct a recombinant RB51 strain that overexpresses the identified Brucella protective protein(s) and simultaneously expresses low amounts of O antigen in its cytoplasm. The recombinant RB51 strain will then be rendered non-replicative by gamma-irradiation and tested for its ability to provide superior protective immunity against all 3 virulent Brucella species infections. Under Specific Aim 3, we will evaluate the efficacy and safety of the non-replicative, brucellosis vaccine in mice following different immunization routes and regimens that facilitate the development of mucosal and systemic protective immune responses. Successful completion of the proposed studies will result in a non-replicative, strain RB51-based human brucellosis vaccine that could be tested for its efficacy and safety in large animal/nonhuman primate models. Relevance: Brucellosis is an important zoonotic disease caused by the bacteria belonging to genus Brucella. Three of the Brucella species that are highly virulent to humans are considered potential bioterror agents. At present there is no vaccine available for prevention of brucellosis in humans. Research proposed in this application focuses on developing a highly effective and safe Brucella vaccine for human use.